Psicose inhibits lettuce root growth via a hexokinase‐independent pathway

Fructose analog, psicose, and glucose analog, mannose, inhibited root growth of lettuce seedlings. Psicose is phosphorylated by hexokinase and fructokinase (EC 2.7.1.4) to psicose‐6‐phosphate with no known capacity for further metabolism. Mannose is phosphorylated by hexokinase (EC 2.7.1.1) to mannose‐6‐phosphate which is further metabolized very slowly. Hexokinase is known to have a sugar‐sensing function and possibly triggers a signal cascade resulting in changes of several gene expressions. It was determined, compared with the behaviour of mannose, whether psicose inhibits the root growth through this system. The addition of phosphate into the growth medium of lettuce seedlings did not affect the inhibition by psicose and mannose, and both sugars did not reduce adenosine triphosphate (ATP) level in the roots, suggesting that the inhibition is not due to phosphate starvation and ATP depletion. The inhibiting effects of psicose and mannose were overcome by adding sucrose into the medium, which suggests that the inhibition is not caused by accumulation of psicose‐6‐phosphate or mannose‐6‐phosphate in the seedlings. Mannoheptulose, a specific competitive inhibitor of hexokinase, defeated the mannose‐induced inhibiting but was not able to relieve the psicose‐induced inhibition. Thus, the phosphorylation of mannose by hexokinase may trigger a signal cascade resulting in the growth inhibition of lettuce roots, which is consistent with the hypothesis established in Arabidopsis . However, psicose cannot inhibit the growth of lettuce roots via a hexokinase‐mediated pathway, and the phosphorylation of psicose by fructokinase might trigger a hexokinase‐independent signal cascade resulting in the growth inhibition.